To fully exploit the bandwidth promised by fiber-optic transmissions, it is necessary to build all-optical networks where optical signals are not converted into an electrical form, except at ingress or egress nodes. In circuit-switched wavelength division multiplexing networks, all-optical routing is provided by all-optical cross-connects capable of switching individual wavelength channels. Circuit-switched wavelength division multiplexed networks share some of the characteristics of traditional circuit-switched networks. However, major differences appear when some cross-connects are not capable of converting frequencies. When this situation occurs, traditional routing algorithms must be modified, and wavelength-continuity constraints must be considered (see B. Ramamurthy and B. Mukherjee, “Wavelength-conversion in WDM networking”, IEEE Journal on Selected Areas on Communications, vol. 16, pages 1061–1073, September 1998). These constraints impose that on nodes without wavelength conversion capability, incoming wavelength channels must be switched to outgoing channels on the same frequency. Wavelength continuity constraints lead to degradations of network blocking performance, and they increase the complexity of bandwidth allocation protocols. These constraints can be removed by using fully-convertible nodes. In transparent all-optical networks, all-optical wavelength converters must be used. However, high costs of all-optical converters often limit the utilization of fully wavelength-convertible nodes, as the number of wavelengths and the size of the network grow. For this reason, much effort has been devoted to routing strategies that optimally use limited wavelength conversion resources (see B. Ramamurthy et al. referenced above).
To address the same problem, another approach is to try and design fully-convertible cross-connects using a minimum number of all-optical converters (see N. Antoniades, S. Yoo, K. Bala, G. Ellinas, and T. Stern, “An architecture for a wavelength-interchanging cross-connect utilizing parametric wavelength-converters”, IEEE Journal of Lightwave Technology, vol. 17, pages 1113–1125, July 1999). Before the development of wave-mixing wavelength converters, this approach has inevitably lead to solutions that assigned a dedicated full-range wavelength converter to each wavelength channel on each fiber link.
With these previous approaches, strictly non-blocking fully-convertible nodes require a number of converters equal to the total number of wavelength channels (in all the fibers). A common characteristic of these first designs is to convert channels through a single wavelength conversion operation, usually carried out at the inputs or at the outputs of a space switch. For this class of solutions, the problem lies in high converter costs. Indeed, for a cross-connect with F fibers and W frequencies per fiber, these solutions require as many as F.W dedicated all-optical converters.
Wave-mixing converters and their bulk wavelength conversion abilities offer new options for designing wavelength-interchanging cross-connects. One particular technique to achieve this goal has been proposed, wherein a wavelength-to-space transformation is exploited (see N. Antoniades et al. referenced above; also see R. Thompson and D. Hunter, “Elementary photonic switching modules in three divisions”, IEEE Journal on Selected Areas in Communications, vol. 14, pp. 362–373, February 1996). In the transformation, channels are simultaneously switched in the space and in the wavelength-domains at each stage. Unlike previous techniques based on single-stage wavelength conversion, this approach converts individual wavelength channels by decomposing these conversions into several elementary wavelength conversions operations (see N. Antoniades et al. referenced above). Also, in this design, the resulting cross-connect architecture is rearrangeably non-blocking and only uses a number of wave-mixing converters equal to half the total number of wavelength channels (see N. Antoniades et al. referenced above). In other words, for a cross-connect with F fibers and W frequencies per fiber, this solution only requires a total of F.W/2 wave-mixing converters (see N. Antoniades et al. referenced above), instead of F.W converters, as would be obtained with other approaches (see B. Ramamurthy et al. referenced above).
In spite of its merits, this design is only rearrangeably non-blocking. Therefore, when it is used, ongoing connections may have to be rearranged to switch new requests. However, high traffic volumes make it difficult to reroute existing lightpaths, without incurring severe QoS degradation. It is possible to build strictly non-blocking cross-connects by combining this solution with the technique of vertical replication (see A. Pattavina, Switching Theory, Wiley, 1998). Yet, when doing so, more converters are required than by other designs based on dedicated wavelength converters.
Another problem of this transformation solution is that it is not adapted to provide a gradual deployment of wavelength conversion (see N. Antoniades et al. and R. Thompson et al. referenced above). Indeed, current semiconductor technology makes it possible to build large wavelength-selective cross-connects at low costs. However, all-optical wavelength converters are in their early development stages, and are still produced at high costs. In a metropolitan network environment, service providers would prefer to start with a simple wavelength-selective cross-connect to minimize initial costs. They would then have the flexibility to upgrade the all-optical wavelength conversion capabilities when special needs appear.
In view of the foregoing, it would be desirable to provide a technique for interchanging wavelengths in a multi-wavelength system in an efficient and cost effective manner which overcomes the above-described inadequacies and shortcomings.